BLASTULA, GASTRULA, AND GERM LAYERS 355 



rudiments of the embryo may be said to be fully marked out. 

 The process of concrescence is seen most clearly and typically 

 in those forms with large amounts of yolk and with well-marked 

 germ ring, especially in the Teleosts and Elasmobranchs (Figs. 

 160, 165). In the Amphibia, where the amount of yolk is less, 

 and the Sauropsida, where the germ ring is less marked, the proc- 

 ess of concrescence, though somewhat modified and slightly 

 obscured, still takes an important part in embryo formation. 



THE GERM LAYERS 

 While this is not the place to give an historical or critical 



FIG. 165. Blastoderms of the Elasmobranch, Torpedo, showing formation of 

 the embryo. After Ziegler. X 27. A. "Stage B." The postero-median 

 thickening is the "embryonic shield," the first indication of the real embryo. 

 B. "Stage C." Early embryo; nerve cord rising above the surface of the blasto- 

 derm. In both figures the embryonic portion of the blastoderm is directly 

 continuous postero-laterally, with the germ ring, which appears as the thickened 

 margin of the blastoderm. 



account of the germ layer theory, it is important that the stu- 

 dent should have in mind, before taking up the study of the 

 development of particular organisms, certain fundamental 

 conceptions of the germ layers, and their relation to develop- 

 ment, particularly among the Chordata. 



When the science of Embryology was itself in a very early 

 stage of its development, the earliest differentiations recognized 

 by the students of animal development, were the sheets or 



